Surgical procedures requiring cutting of tissue can cause bleeding at the site of the cutting. Various techniques have been adapted to control bleeding with varying degrees of success such as, for example, suturing, applying clips to blood vessels, and stapling, as well as electrocautery and other tissue heating techniques. Advances in tissue joining or welding, tissue repair and wound closure also have permitted surgical procedures previously not possible or too risky.
Surgical staplers have been used for tissue security, joining and approximation, and to provide hemostasis in conjunction with tissue cutting. Such devices include, for example, linear and circular cutting and stapling instruments. Typically, a linear cutter has parallel rows of staples with a slot for a cutting means to travel between the rows of staples. This type of surgical stapler secures tissue for improved cutting, joins layers of tissue, and provides hemostasis by applying parallel rows of staples to layers of surrounding tissue as the cutting means cuts between the parallel rows.
Electrocautery devices have been used for effecting improved hemostasis by heating tissue and blood vessels to cause coagulation or cauterization. Monopolar devices utilize one electrode associated with a cutting or cauterizing instrument and a remote return electrode, usually adhered externally to the patient. More recently, bipolar instruments have been used because the cauterizing current is generally limited to tissue between two electrodes of a tissue treating portion of an instrument.
Bipolar forceps have been used for cutting and/or coagulation in various procedures. Generally, bipolar forceps grasp tissue between two poles and apply electrical current through the grasped tissue. Bipolar forceps, however, have certain drawbacks, some of which include the tendency of the current to arc between poles when tissue is thin or the forceps to short when the poles of the forceps touch. The use of forceps for coagulation is also very technique dependent and the forceps are not adapted to simultaneously cauterize a larger area of tissue. Furthermore, forceps tend to cause areas of thermal spread, i.e., dissipation of heat outside of area defined by grasping or engaging surfaces of the forceps.
When using RF energy in electrosurgical devices, there may be an optimal range of tissue impedances that results in the best or optimal energy delivery for the output characteristics of the particular generator to which the impedance load is presented.
Generally, the optimal range is related to the principal that where the source and load impedances are matched, the transfer of power from the source to the load is maximized. Further, the power output for a given generator decreases at a predictable rate as impedance of the load, i.e., tissue, falls off of the source impedance.
The optimal range is defined herein as the range of load impedances at which the power transfer from the generator is sufficient to achieve the intended result, i.e., controlled coagulation, cauterization, or tissue welding. The optimal range may vary from application to application or from generator to generator.
It is believed that tissue impedance varies depending on a number of parameters which may include: tissue type, liquid content, tissue condition (i.e., coagulated or uncoagulated), tissue thickness, the amount of tissue compression, the size and length of the flow path of electrical current through the tissue, and energy frequency applied to tissue.
Additionally, for a given area or volume of tissue,-the impedance model of the tissue is dynamic due to the fact that tissue impedance changes as tissue is heated and begins to coagulate, thus effecting the current flow pattern through the area or volume of tissue as coagulating current is delivered to the tissue. It is also believed that the tissue thickness typically changes as it is electrosurgically treated, because, for example, as water escapes in the form of steam or vapor the volume of the material grasped by the instrument is reduced. Depending on the specific end effector configuration, this could provide an additional variable in the impedance model of the tissue if in the particular end effector tissue thickness were to effect the length of the current path and/or the amount of compression applied to the tissue.
Thus, it is desirable to provide an electrosurgical device which can efficiently provide hemostasis in multiple tissue types and thicknesses, e.g., in fleshy or vascular tissue areas, and high, low or combination impedance tissues. Hemostasis is used herein, generally, to mean the arresting of bleeding including by coagulation, cauterization and/or tissue joining or welding.
It is further desirable to provide a device which adapts to the changing impedance and/or tissue thicknesses as the tissue is being treated, so that the impedance presented to the generator is within an optimal range.